This invention relates to a thermosetting resin composition. More particularly, it relates to a thermosetting composition which is suitable to form on the surface of plastic materials a coating that remarkably improves abrasion resistance, hardness, heat resistance, chemical resistance, weathering resistance, etc. of the plastic surface. This composition can be applied on the surface of many materials including papers and metals.
Among the plastic materials in wide use today, acrylic and methacrylic resins are superior in mechanical properties and appearance and clarity. However, these resins are defective in that they are inferior in abrasion resistance and surface hardness. As a resin that is provided with rather high abrasion resistance, diethyleneglycol bis(allyl carbonate) polymer (CR-39) is known. But the abrasion resistance and surface hardness of this resin are not quite satisfactory in practical use.
Therefore many methods for improving surface hardness of acrylic and methacrylic resins have been proposed and studied. For instance in Japanese Patent Application Publication No. 17847/60, a method is disclosed in which a cross-linking monomer such as triethyleneglycol dimethacrylate or an alkanediol dimethacrylate is applied on the inside surface of a casting mold as a film and thereafter methyl methacrylate is cast-polymerized in said mold. In Japanese Patent Application Publication No. 9827/62, another method is disclosed in which a cross-linking monomer such as allyl methacrylate, ethylene dimethacrylate, etc. is applied on the inside surface of a casting mold and a resin article which has been formed by the same mold beforehand is placed therein and the mold is heated so as to polymerize the applied monomer. In Japanese Patent Laying-Open Publication No. 26419/72, a method is disclosed in which a solution prepared by mixing a hydrolysate of an alkyl silicate such as butyl silicate and a copolymer of vinyl triethoxysilane and vinyl acetate for instance in a solvent is applied on the surface of methacrylic resin plates and the plates are heated so as to cure the applied material.
In Japanese Patent Application Publication No. 34707/70, a surface coating composition is disclosed. Said composition comprises 3 - 30% aminoplast, 0.5 - 10% an epoxy-containing alkoxysilane and the balance is a hydroxyalkyl ester of vinyl-group-containing unsaturated carboxylic acid. This composition is applied on the surface of metal plates and cured by heating.
However, all these methods and compositions are inadequate and insufficient for hardening the surface of clear and transparent plastic materials. The methods of Japanese Patent Application Publication No. 17847/60 and Japanese Patent Application Publication No. 9827/62 are unsatisfactory in that a long time is required for curing of the coating or film and the abrasion resistance of the formed coating is not sufficiently high. The coating provided by the method of Japanese Patent Application Publication No. 34707/70 is not transparent and not so hard, and therefore improvement in surface hardness and abrasion resistance of plastic materials by coating cannot be expected from this method. The method disclosed in Japanese Patent Laying-Open Publication No. 26419/72 has a fatal defect in heat resistance and weathering resistance of the formed coating. That is, the coating formed on the surface of methyl methacrylate resin wrinkles within several minutes when heated over 80.degree. C. and the inherent optical characteristics of the substrate resin are spoiled. Also this method cannot be applied to some kinds of plastic materials, for instance, this method cannot be employed for a resin such as CR-39.
In order to overcome the defects of the prior art as mentioned above, we studied the use of silicon-containing polymers as a coating for plastic materials, and discovered a composition suitable for forming a coating film on the surface of the plastics. Said composition gives a coating excellent in transparency, abrasion resistance, surface hardness, heat resistance, weathering resistance, chemical resistance and adhesion to substrate resins.